1. Field of the Invention
The invention relates to a magnetoresistive device having at least a magnetoresistive film, a thin film magnetic head using at least the magnetoresistive device and methods of manufacturing the same.
2. Description of the Related Art
In recent years, performance improvement in thin film magnetic heads has been sought in accordance with an increase in surface recording density of a hard disk drive. As a thin film magnetic head, a composite thin film magnetic head has been widely used. A composite thin film magnetic head has a layered structure which includes a recording head with an inductive-type magnetic transducer for writing and a reproducing head with a magnetoresistive device (also referred as MR device in the followings) for reading-out. There are a few types of MR devices: one is an AMR device that utilizes the anisotropic magnetoresistance effect (referred as AMR effect in the followings) and the other is a GMR device that utilizes the giant magnetoresistance effect (referred as GMR effect in the followings). A reproducing head using an AMR device is called an AMR head or simply an MR head. A reproducing head using the GMR device is called a GMR head. The AMR head is used as a reproducing head whose surface recording density is more than 1 gigabit per square inch. The GMR head is used as a reproducing head whose surface recording density is more than 3 gigabit per square inch.
The AMR head includes an AMR film having the AMR effect. The GMR head has the similar configuration to the AMR head except that the AMR film is replaced with a GMR film having the GMR effect. However, compared to the AMR film, the GMR film exhibits a greater change in resistance under a specific external magnetic field. Accordingly, the reproducing output of the GMR head becomes about three to five times greater than that of the AMR head.
An MR film may be changed in order to improve the performance of a reproducing head. In general, an AMR film is a film made of a magnetic substance which exhibits the MR effect and has a single-layered structure. In contrast, most of the GMR films have a multi-layered structure consisting of a plurality of films. There are several types of mechanisms which produce the GMR effect. The layer structure of the GMR film depends on those mechanisms. GMR films include a superlattice GMR film, a granular film, a spin valve film and so on. The spin valve film is most sufficient since the film has a relatively simple structure, exhibits a great change in resistance in a low magnetic field, and is suitable for mass production. The performance of a reproducing head is thus easily improved by replacing an AMR film with a GMR film and the like with an excellent magnetoresistive sensitivity.
As a primary factor for determining the performance of a reproducing head, there is a pattern width, especially an MR height, in addition to selection of materials as described. The MR height is the length (height) between the end of an MR element closer to an air bearing surface (the surface to be faced with the recording medium) and the other end. The MR height is originally determined by an amount of grinding when the air bearing surface is processed.
As described, the performance of a reproducing head can be easily improved by replacing the AMR film with the MR film made of a material with better magnetoresistive sensitivity such as the GMR film or the like. The GMR films, however, are still under development except for a spin-valve GMR film and need more study. Accordingly, it has been tried to manufacture a reproducing head which is applicable to a higher surface recording density such as 3-4 gigabit inches per square or more while using the AMR film of the related art.
Incidentally, the MR film is not used by itself in general, but is used in combination with another magnetic film for applying a lateral magnetizing bias field to the MR film. As used herein, the lateral magnetizing bias field is a magnetizing bias field in a direction orthogonal to the direction of sense current which is applied to the MR film for sensing resistance change of the MR film. The magnetizing bias field is applied so as to perform reproduction using the linear region among the resistance-change curve of the MR film (curve in which the relation of magnetic field H applied from outside and change in resistance .rho. of the MR film is plotted). Methods for applying the lateral magnetizing bias field include self biasing, shunt biasing, soft-film biasing and so on, as introduced in "THE JOURNAL OF THE INSTITUTE OF IMAGE INFORMATION AND TELEVISION ENGINEERS" Vol. 51, No. 6, issued in June 1997, published by THE INSTITUTE OF IMAGE INFORMATION AND TELEVISION ENGINEERS.
The first one of these techniques, the self-biasing includes the steps of: placing the MR film closer to one of magnetic separation layers (shield layer) which sandwich the MR film; applying sense current to the MR film in order to magnetize the adjacent magnetic separation layer by the generated magnetic field; and applying the magnetic field generated from the magnetized magnetic separation layer to the MR film as the magnetizing bias field. The second one, the shunt biasing is, as disclosed in, for example, Japanese Patent Application laid-open Sho 61-253620, a method in which a nonmagnetic conductive thin film is placed adjacent to the MR film and the magnetic field generated by current flowing in the nonmagnetic conductive thin film is used as the magnetizing bias field. The third one, the soft-film biasing includes the steps of: placing a soft magnetic film called SAL (Soft Adjacent Layer) adjacent to the MR film with the magnetic shield film in between; applying sense current to the MR film in order to magnetize the adjacent magnetic separation layer by the generated magnetic field; and applying the magnetic field generated from the soft film to the MR film as a lateral magnetizing bias field, as disclosed in, for example, Japanese Patent Application laid-open Sho 50-65213.
Among the techniques described above, the soft-film biasing is in the most widespread use. In this method, originally, the film made of tantalum (Ta) which is conductive is mostly used as the magnetic shield film to be inserted between the MR film and the soft film. In such a case, however, sense current flows not only into the MR film but also into other films (that is, magnetic shield film and soft film). This causes the current flowing in the MR film to become less so that reproducing output is decreased. A method to overcome this problem is to insulate the MR film and the soft film electrically by using an insulating film made of alumina (Al.sub.2 O.sub.3) or other materials as the magnetic shield film so that the sense current only flows into the MR film. This method contributes to prevent decrease in reproducing output since, as a rule, all the sense current flows into the MR film without flowing in the soft film. However, if there is a short-circuit due to incomplete insulation between the MR film and the soft film, improving the reproducing output as described above is not achieved. Accordingly, a technique for preventing a short-circuit between the MR film and the soft film in the manufacturing procedure is disclosed in, for example, Japanese Patent Application laid-open Hei 7-320235.
Now, the principal point of a method of manufacturing a thin film magnetic head of the related art with the MR device composed of a SAL film, a film made of alumina and the MR film will be described with reference to FIG. 26 to FIG. 30. Further, FIG. 26 to FIG. 30 show enlarged cross sections parallel to the air bearing surface of the reproducing head.
In the manufacturing method of the related art, first, as shown in FIG. 26, an insulating layer 102 made of, for example, alumina is deposited on a substrate 101 made of, for example, altic (Al.sub.2 O.sub.3 --TiC). Further, a bottom shield layer 103 for a reproducing head made of a magnetic material is formed on the insulating layer 102. Next, a bottom shield gap film 104 as an insulating layer is formed by depositing, for example, alumina on the bottom shield layer 103.
Next, an MR laminated film 105' is formed by laminating a SAL film 105a', an insulating film 105b' and an MR film 105c' in order on the bottom shield gap film 104. The SAL film 105a' is formed as a soft magnetic film and the insulating film 105b' is formed using, for example, alumina while the MR film 105c' is formed using, for example, permalloy (Ni--Fe). Further, in FIG. 26, the SAL film 105a', the insulating film 105b' and the MR film 105c' are drawn thicker than as they actually are compared to the film of other layers.
Next, as shown in FIG. 27, a photoresist pattern 106a with a T-shaped cross section for simplifying lift-off, which is to be performed later, is selectively formed in a position on the MR laminated film 105', where an MR device 105 is to be formed. Next, a pattern of the MR device 105 composed of a SAL film 105a, an insulating film 105b and an MR film 105c is formed by etching the MR film 105c', the insulating film 105b' and the SAL film 105a' composing the MR laminated layer 105' in order. The etching is performed by oblique ion milling method with the photoresist pattern 106a being a mask. In the ion milling process, ion beam is irradiated to the MR film 105c' from the oblique direction. As a result, the side-end faces of the SAL film 105a, the insulating film 105b and the MR film 105c slope toward the bottom shield gap film 104.
Next, as shown in FIG. 28, an insulating film 105d made of, for example, alumina is formed to a thickness of about 10 nm by using the photoresist pattern 106a as a mask which is used at the time of forming the MR device 105. Through this, each of the side-end faces of the SAL film 105a, the insulating film 105b and the MR film 105c composing the MR device 105 are covered with the insulating film 105d. Next, the photoresist pattern 106a is removed by lift-off together with the unwanted part of the insulating film 105d.
Next, as shown in FIG. 29, a photoresist pattern 106b is formed substantially in the center of the MR film 105c of the MR device 105. The photoresist pattern 106b is also formed so that its cross section takes a T-shape like the photoresist pattern 106a. The width of the photoresist pattern 106b is, however, made smaller than that of the photoresist pattern 106a. A pair of first electrode layers 107, which are electrically connected to the MR device 105 as an lead electrode, are formed on the bottom shield gap film 104 with the photoresist pattern 106b being a mask. At this time, the first electrode layers 107 are insulated from the side-end faces of the MR device 105 by the insulating film 105d covering the area from the bottom shield gap film 104 to the side-end faces of the MR device 105, while being electrically connected only to the MR film 105c' to part of the top surface at both ends of the MR film 105c of the MR device 105.
Next, as shown in FIG. 30, after the photoresist pattern 106b is removed by lift-off together with the unwanted part of the first electrode layers 107, a pair of second electrode layers, not shown in figure, which are electrically connected to the first electrode layers 107 respectively in areas in the back of the paper, are formed to a predetermined pattern. The first electrode layers 107 and the second electrode layers compose a lead electrode which is electrically connected to the MR device 105. Next, a top shield gap film 108 as an insulating layer made of such as aluminium nitride is formed to cover the bottom shield gap film 104 and the MR device 105, and the MR device 105 is buried between the shield gap films 104 and 108. Next, a top shield layer which also works as the bottom pole (referred as top shield layer in the followings) 109, which is made of a magnetic material and is used both for the reproducing head and for the recording head, is formed on the top shield gap film 108.
The process of forming the portion of the reproducing head in a composite thin film magnetic head of the related art is hereby completed. The process of forming the portion of the recording head is carried out thereafter.
As described, in the method of manufacturing a thin film magnetic head of the related art, the MR film 105c', the insulating film 105b' and the SAL film 105a' composing the MR laminated film 105' are etched at one step by ion milling method in the step of forming the MR device 105 composing the reproducing head. The insulating film 105b', which is the middle layer, has a lower etching rate compared to that of other two layers (SAL film 105a' and MR film 105c') so that the side-end faces of the insulting film 105b does not have the same slant as that of other two layers. Further, the side-end portion of the insulating film 105b remains protruded from between the side-end faces of other two layers taking an overhang shape. As a result, as shown in FIG. 31, the insulating film 105d is to have pin holes PH which are formed below the overhang portion, and the insulating ability of the insulating film 105d changes if there exist the pin holes PH. If the insulating film 105d of the manufactured head has the pin holes PH, current may flow into the SAL film 105a and the amount of the current flowing to the MR film 105c decreases. Accordingly, there is a problem that the reproducing output characteristic varies among reproducing heads.
Japanese Patent Application laid-open Hei 7-320235 mentioned above discloses a method in which knoop hardness of the soft film is increased in order to suppress short-circuit in the MR film and the soft film, which is likely to occur in the step of polishing the air bearing surface in the manufacturing procedure. However, there is no consideration disclosed for the problem as mentioned above, that is, the reproducing output varies among heads due to the difference in the insulating ability of the thin insulating film provided between the lead electrode layer (first electrode layer 107) and the soft magnetic film (SAL film 105a).
The invention is designed to overcome the foregoing problems. An object of the invention is to provide a magnetoresistive device which enables a stable operation with less variations in output obtained through magnetoresistance, a thin film magnetic head formed by using at least such a magnetoresistive device, which can perform stable operation and methods of manufacturing the same.